A surface of such an underwater structure as marine vessels and power generating plants is painted with an antifouling paint in order to prevent adhesion of such aquatic organisms as acorn barnacle, mussels, and algae. Since the underwater structures painted with the antifouling paint are normally large in size, painting with an antifouling paint is often carried out by using a crane truck or the like. It is not easy, however, to paint the entire surface of an underwater structure having a large area uniformly to a prescribed film thickness.
An antifouling paint has a prescribed dry coating film thickness range (in particular, a minimum coating film thickness) necessary for exhibiting expected coating film performance (such as antifouling performance). For example, when a thickness is smaller than the prescribed minimum coating film thickness and even though a portion where a film thickness is insufficient is a part of the entire surface, expected antifouling performance cannot be provided to an underwater structure and an effective antifouling period is virtually shortened.
In order to address such a problem, for example, Japanese Patent Laying-Open No. 2000-005692 (PTL 1) discloses a method of forming an antifouling coating film by using such a film thickness determinable antifouling paint characterized in that a color difference between the film thickness determinable antifouling paint and an object to be coated, a color difference between a completely hiding coating film with a target dry film thickness and a coating film having a dry film thickness less than (target dry film thickness−50) μm, and a color difference between the completely hiding coating film with the target dry film thickness and a coating film having a dry film thickness exceeding (target dry film thickness+50) μm are each within a prescribed range and that a content of an antifouling agent and a color pigment is not greater than a prescribed value. According to such a film thickness determinable antifouling paint, whether the coating film has reached a prescribed film thickness or not can be determined by visually observing change in color difference between the coating film being applied and the object to be coated, and hence painting with the antifouling paint can be carried out substantially uniformly in a simplified manner, without excess or shortage in coating film thickness.
The film thickness determinable antifouling paint described in PTL 1 above, however, is relatively small in color difference between the coating film having the target dry film thickness and the coating film immediately before reaching the target dry film thickness, and therefore it may sometimes be difficult to determine whether a coating film being applied has reached a prescribed film thickness or not by visual observation by a paint operator, and there has been a room for improvement.